How do I know if my customer is eligible for nonprofit price?

The partner will have to inquire with a customer about whether they have registered with Microsoft directly and possess office 365 nonprofit account. In case if the customer cannot confirm this, you can direct the customer to login to nonprofit portal here: https://nonprofit.microsoft.com/#/signin. If the customer can login successfully to the portal then they have a valid nonprofit tenant. You can then transition the customer tenant to rhipe CSP and NFP subscriptions to the tenant.